The present invention generally relates to diffusion brazing processes and materials for components that operate at high temperatures. More particularly, this invention relates to a process of filling blind holes, through-holes, and cavities in castings, such as hot gas path components of gas turbines.
Components of gas turbines, such as buckets (blades), nozzles (vanes), and other hot gas path components, are typically formed of nickel, cobalt or iron-base superalloys with desirable mechanical properties for turbine operating temperatures and conditions. Because the efficiency of a gas turbine is dependent on its operating temperatures, there is a demand for components, and particularly turbine buckets and nozzles, that are capable of withstanding increasingly higher temperatures. As the maximum local metal temperature of a superalloy component approaches the melting temperature of the superalloy, forced air cooling becomes necessary. For this reason, airfoils of gas turbine buckets and nozzles often require complex cooling schemes in which air is forced through internal cooling passages within the airfoil and then discharged through cooling holes at the airfoil surface.
Buckets and nozzles formed by casting processes require cores to define the internal cooling passages. During the casting process, shifting of the cores is prevented by supporting the cores within the mold using quartz rods or similar means. The rods create openings (through-holes) in the casting that must be securely closed or plugged to prevent the loss of cooling air through these holes and ensure proper air flow levels through the intended cooling holes of the casting. Various methods have been used to fill these holes, including brazing and welding techniques, the latter of which includes tungsten inert gas (TIG) welding, electron beam welding, and laser beam welding. In some cases, welding is not practical for closing or filling holes resulting from casting operations due to costs, poor fusion weldability of the material, inaccessibility with welding equipment, and other restrictions arising from the configuration of the component. Furthermore, welding techniques involve application of localized heat energy that produces a fusion zone and a base metal heat affected zone (HAZ) that are prone to liquation and strain age cracking. Brazing techniques are generally performed at temperatures lower than the melting point of the base metals and, when performed appropriately, are not susceptible to cracking.
Brazing performed on superalloy castings have typically involved the use of braze materials in pliable forms such as pastes, putties, slurries, and tapes, as evidenced by commonly-assigned U.S. Pat. No. 6,187,450 to Budinger et al., U.S. Pat. No. 6,530,971 to Cohen et al., and U.S. Pat. No. 7,279,229 to Budinger et al. Brazing techniques using sintered preforms have also been proposed for applying wear resistant materials on bucket surfaces, as taught in commonly-assigned U.S. Pat. No. 7,335,427 to Sathian, and for surface buildup and hardfacing as taught in commonly-assigned U.S. Published Patent Application No. 2007/0154338 to Sathian et al.
Brazing pastes, putties, slurries, and tapes generally contain metal particles in a binder that adheres the metal particles together and to the surface(s) being brazed, and then burns off during the brazing operation. The metal particles are typically a mixture of two or more different alloys, one of which contains a melting point depressant (for example, boron or silicon) to achieve a lower melting point. During brazing, only the lower melting particles melt to form a liquid that fills voids between the higher melting particles and, on solidification, bonds the high melting particles together and to the substrate material. Shortcomings associated with the use of such pliable braze materials include the difficulty of consistently using optimal quantities of the braze material, accurately placing the braze material, and accurately shaping and sizing the braze material for the area being brazed. Because of their pliability, which includes the ability to flow in the case of slurries, other shortcomings include the difficulty of filling large openings and surfaces where the braze material is likely to flow away from the area being brazed. Still other shortcomings typically include low densities and excessive porosity and voids, resulting in poor mechanical properties for the resulting brazement.
Sintered brazing preforms may also be initially prepared to contain a binder, which is removed during a sintering operation performed prior to brazing, resulting in the metal particles being sintered together (fused or agglomerated) to yield a rigid preform. Because of their rigidity, sintered preforms have been generally limited to surface repairs, as suggested by the above-identified patent documents to Sathian and Sathian et al.